The conditioning (sweetening) of gases, natural and synthetic, i.e., acid gas stripping, the removal of acidic gases, e.g., CO.sub.2, H.sub.2 S and COS, by absorption of the acidic gases in a liquid absorbent medium, has been practiced commercially for many years. Various absorbents, such as the alkanolamines, "sulfolane" (tetrahydrothiophene-1,1-dioxide), "sulfinol" (tetrahydrothiophene-1,1-dioxide plus diisopropanolamine), potassium carbonate and the like, have been used commercially. Each of these systems is plagued by corrosion problems, some of which result from decomposition of the absorbent, some from reaction between the acidic components of the gases treated and the absorbent, and all, to more or less the same degree, from attack by the acidic components of the gases treated upon the metals of construction of the equipment. Generally, the corrosion occurs in the regenerator, heat exchangers, pumps and piping associated with these elements of the overall equipment. Numerous compounds have been suggested as additives to the absorbents to prevent the corrosion and/or the formation of corrosive elements. For example, copper sulfate was used for three years in a 15 percent monoethanolamine gas processing plant for removing 10 percent CO.sub.2 from the gas. Corrosion was observed as a decrease in reboiler and heat exchange tube life and on analysis of the amine solution, only a few parts per million copper was found thus indicating excessive copper deposition in the unit. (Gas Conditioning Fact Book, The Dow Chemical Company, Midland, Mich. 1962, pp. 157-158, Case Number 8). In another situation, described in U.S. Pat. No. 2,377,966, copper sulfate or copper carbonate in an aqueous alkanolamine solution was used as the absorbent to remove CO.sub.2 from a gas stream in which no H.sub.2 S was present. The copper compound in the absence of any H.sub.2 S was used to reduce the iron content of the solution (iron content being a measure of the oxidation of the solution and metal) thus indicating a reduction in the corrosivity of the solution to iron. The alkanolamine with the absence of H.sub.2 S reacted in the presence of the copper to yield oxalic acid after going through aminoacetic acid, which apparently is a product of the oxidation of the amine in the presence of iron. In still a further literature, reference, U.S. Pat. No. 2,771,405, copper corrosion by sulfur compounds and/or free-sulfur is reduced by solvent extracting the petroleum distillates with aqueous alkanolamine to remove the sulfur. Thus, in practice, copper has not been satisfactory as an inhibitor in acid-gas stripping plants.
U.S. Pat. No. 1,989,004, Fife, teaches using an alkanolamine or mixture of mono- , di- and trialkanolamines in a concentration of 15 to 30 percent in water and less than 1 percent of a soluble metal salt, e.g., copper or nickel sulfate or oxide.
This does not teach maintaining sulfur, that is, elemental or the necessary sulfur compounds and oxidizing agents to produce sulfur atoms.
Fife absorbs the H.sub.2 S and organic sulfur compounds from the gas using the complexing activity of each of the components, viz., the amine, the amine-metal complex and the metal. These complexes are nonthermally treated in two steps to release the amine for reuse and convert the sulfur to a solid for removal from the system.
Fife treats the spent solution (aqueous alkanolamine containing the amine-sulfur compound complex and the amine-metal sulfur compound complex) not with heat to regenerate but with air (page 1, column 2, lines 20-38) to oxidize the sulfur compounds (thiosulfates) and sulfates, then the oxidized solution is treated with lime to precipitate the sulfur as calcium sulfate and thiosulfates. It is well known that the alkanolamine also undergoes oxidation and that aeration would increase the loss of alkanolamine as is evidenced by the fact that the 15 to 30 percent amine solution used in Example II (page 2, column 1, lines 49-56) after regeneration constituted only a 10 to 13 percent amine solution, a loss of about 30 percent of the original amine.
U.S. Pat. No. 3,372,981, Ravner, discloses the use of copper to stabilize monoethanolamine solution against degradation during use as an absorbent for CO.sub.2 only from air (no H.sub.2 S present). Ravner uses iron, copper and a chromium salt but uses no sulfur.
U.S. Pat. No. 2,559,580, Alexander, discloses that iodine must be present in an alkanolamine solution in order to prevent degradation of the amine in which copper is present. This reference teaches that copper without iodine degrades amines, column 5, lines 19-59.
U.S. Pat. No. 2,405,672, Reitmeier, teaches the use of dichromates and a metal salt, e.g., zinc, copper, cadmium, mercury, arsenic, etc., in amounts of 2 to 3 percent. The metal salt combines with the H.sub.2 S to form an insoluble sulfide which precipitates and is thus available for oxidation by the dichromate. The pH of the reference solution must be controlled to be sure the CuS precipitates so that it is available for reaction with the dichromate to form the sulfur (column 3, line 38 to column 4, line 13). This reference requires no amine.
In recent years, both natural and synthetic gases are being produced which contain high concentrations of CO.sub.2 and/or H.sub.2 S. As the demand for neutral gases increases, the size of the units for treating these gases increases, thus, economically an increase in the concentration of absorbent in the system and/or an increase in the loading on the systems seems desirable. Both of the latter, although most desirable, increase the corrosion in the unit resulting in more frequent unscheduled downtimes for repair of and replacement of major elements.
It is therefore an object of the present invention to provide an inhibitor system for use in equipment in contact with acid-gas environments which reduces the corrosion of the metals of construction of equipment used under the conditions.
It is a further object to provide a process which will reduce the economic loss of plant elements and plant operating down-times between normal annual plant turn-around.
A still further object of the present invention is to provide an inhibitor system which will be effective in gas-sweetening processes at loadings in excess of 0.5 mole of acid-gas per mole of absorbent and/or concentrations of absorbents in the circulating solution in excess of 40 percent, thereby reducing plant size, increasing flexibility to operate with and from various gas sources and reduce environmental impact on the surrounding.
These and other objects will become apparent from the description and claims hereinafter set forth.